The present invention relates to fluid flow meters for measuring the rate of fluid flowing through a pipe or conduit, and more particularly, to gas utility meters utilizing a vaned rotor and a non-contacting sensor.
Flow meters of a type commonly used as commercial utility meters include a housing enclosing a vaned rotor mounted on an axle connected to a tachometer. Fluid flow through the meter causes the rotor to rotate at a speed proportional to the flow rate. The tachometer generates a signal proportional to the rotor speed to indicate volumetric flow rate of the fluid through the meter.
A disadvantage with such meters is that a certain amount of leakage occurs from the bore through which the axle extends from the meter body. While this leakage is minimized by packing the axle to seal the bore, this does not solve the problem completely. Furthermore, such packing adds to the cost of the meter, has a limited lifespan, and when it fails can contaminate the liquid or gas flowing through the meter. In addition, the packing may be less effective at relatively high fluid pressures.
In response to such problems, fluid flow meters have been developed which do not require a mechanical connection between the rotor and the tachometer and thus can be completely sealed within the housing. This can be accomplished either by fabricating the rotor blades from a magnetic or conductive material or by mounting a magnetic or conductive element on one or more of the blades of the rotor. A sensor is positioned outside the meter body which generates a signal or pulse in response to the vanes of the rotor passing through a magnetic or electric field produced by the sensor, and a corresponding fluid flow rate is determined from the rate of pulses generated. Typical systems of this type use vane-mounted magnets or aluminum vanes and a sensor which detects a change in magnetic flux caused by a passing vane. Other systems have been developed which use radio frequency sensors. Irrespective of the mechanism employed, pulses received from the sensor are processed to determine the volumetric flow rate. Specifically, the pulses are counted to determine the total volume that has passed through the meter.
While such non-contact systems for measuring fluid flow rate solved many problems encountered with the earlier, mechanically connected flow meters, these systems have disadvantages. Since the accuracy of such magnetic or radio frequency based flow meters depends upon the ability of the sensor to detect a passing rotor through a magnetic or RF field, the body of the flow meter is made of a material which does not alter or distort the field. Furthermore, these flow meters are often subjected to highly corrosive environments and must withstand fluid pressures of up to 1440 psig. In most cases, these requirements have been met by fabricating the meter bodies entirely from non-magnetic stainless steel or other non-magnetic material. However, the use of stainless steel or other materials for the entire meter body increase the material cost, as well as the cost of fabrication. Also, the use of some alternative non-magnetic materials react with the fluid that is being measured, and therefore have an undesirably short useful life.
Attempts have been made to avoid the use of non-magnetic bodies by using a sensor encased in a non-magnetic well. This well is installed in an opening through the body which can be made of less expensive magnetic material. A seal must be provided between the body and sensor. A problem encountered with such prior flow meters is that, should a sensor need to be removed from the housing for repair or replacement, it is necessary to depressurize the entire line where the meter is located, since the sensor module itself forms part of the meter body and its removal creates an opening. In most cases, this requires down time.
Accordingly, there is a need for a flow meter of the non-contacting type which can be made of a conventional, relatively inexpensive and robust material, but will not interfere with the magnetic field detected by the sensor, is corrosion resistant, can withstand high pressures, allows removal of the sensor without requiring depressurization of the fluid line, and can safely be removed without endangering service personnel.